Ecient Fair Exchange with Veriable Conrmation of Signatures
Liqun Chen · 1998
We propose a new ecient protocol, which allows a pair of potentially mistrusting parties to exchange digital signatures over the Internet in a fair way, such that after the protocol is running, either each party obtains the other's signature, or neither of them does. The pro- tocol relies on an o-line Trusted Third Party (TTP), which does not take part in the exchange unless any of the parties behaves improperly or other faults occur. Eciency of the protocol is achieved by using a cryptographic primitive, called conrmable signatures (or designated con- rmer signatures in its original proposal (9)). We recommend using a new ecient conrmable signature scheme in the proposed fair exchange pro- tocol. This scheme combines the family of discrete logarithm (DL) based signature algorithms and a zero-knowledge (ZK) proof on the equality of two DLs. The protocol has a practical level of performance: only a moderate number of communication rounds and ordinary signatures are required. The security of the protocol can be established from that of the underlying signature algorithms and that of the ZK proof used.